Method for purifying melamine

ABSTRACT

The invention relates to a method for purifying melamine in which impure melamine is heated to a temperature range above 325°C. but lower than the melting point of melamine which depends on the prevailing ammonia pressure. In addition, the impure melamine is held in said temperature range under an ammonia pressure ranging from 1 to 150 bar for a duration lasting from 1 minute up to 20 hours. The melamine is provided in solid form. Afterwards, the melamine is relieved from pressure and cooled in any order.

The invention relates to a method for purifying melamine, in which solidmelamine is allowed to dwell under ammonia pressure close to the meltingpoint.

Melamine is preferably produced by pyrolysis of urea, it being possibleto use both low-pressure processes and high-pressure processes, asdescribed, for example, in “Ullmann's Encyclopedia of IndustrialChemistry, Vol. A 16, 5^(th) ed (1990), pages 171-185”. The crudemelamine obtained in the melamine synthesis contains about 94-98% byweight of melamine, depending on the production process, and inparticular melam, melem, melone, ureidomelamine, ammeline and ammelideas substantial impurities and must be further purified for more criticalapplications by additional process steps, such as, for example,recrystallization, or by a special procedure during the isolation, suchas, for example, slow cooling, as described, for example, in WO96/20182,by a special temperature programme before the solidification, asdescribed, for example, in WO96/23778, or by dwelling of the solidmelamine under ammonia pressure, as described, for example, inWO96/20183, or by a combination of these process steps.

It was still the object, however, to find novel process variants whichlead to a further increase in the melamine purity or to obtain purermelamine in a simpler manner, optionally also at pressures which are notso high.

It was unexpectedly found that more impurities are present or are formedduring the dwelling of melamine under ammonia pressure both attemperatures well below the melting point and at temperatures just abovethe melting point than during the dwelling at temperatures in the solidrange just below the melting point.

The invention accordingly relates to a method for purifying melamine,characterized in that impure melamine is brought into a temperaturerange T_(T) above 325° C. but lower than the melting point of themelamine which depends on the respective prevailing ammonia pressure andis kept in this temperature range at an ammonia pressure of from 1 to150 bar for the duration of from 1 minute to 20 hours, the melaminebeing present as a solid in this temperature range, after which, in anydesired order, the pressure is relieved and, if desired, cooling to roomtemperature is effected and pure melamine is obtained in solid form. Thedwelling, according to the invention, of the solid melamine (tempering)is preferably carried out in a temperature range T_(T) between 330° C.(or particularly preferably between 335° C.) and the melting point ofmelamine which depends on the respective ammonia pressure.

The temperature can be kept constant during tempering but it can also bechanged within the temperature range T_(T) described above. Thus, thetemperature can be decreased or increased, for example continuously ordiscontinuously, within the limits of the temperature range T_(T).

According to the invention, the pressure during the tempering ispreferably between 5 and 100 bar, particularly preferably between 10 and50 bar. The dwell time during the tempering according to the inventionis preferably from 6 min to 10 h, particularly preferably from 3 min to5 h. The dwell time depends in particular on the desired purity of themelamine; the longer the duration of tempering, the purer is themelamine obtained. The dwell time is dependent on the respective processconditions. In order to obtain a specific purity, a shorter dwell timeis accordingly sufficient in each case at higher temperatures and athigher pressures.

The purification method (tempering) according to the invention can bepreferably carried out by a procedure in which either impure solidmelamine is heated, or impure liquid melamine is cooled, to thetemperature range (tempering range) T_(T) and is allowed to dwell there.

After the end of the tempering, it is possible, depending on thetechnical conditions, first to cool and then to relieve the pressure or,conversely, first to relieve the pressure and then to cool. These stepscan optionally be carried out in a further reaction apparatus. Melamineis preferably cooled to room temperature, for example with the aid ofheat exchangers, by simple removal of the heating medium or by mixingwith cold gases. By means of the method according to the invention,melamine is obtained in crystalline form or as a powder and has inparticular a substantially reduced content of melam, melem and ammelide,which in some cases is even below the limit of detection.

The method according to the invention can be carried out both batchwiseand continuously.

The melamine purification according to the invention is suitable inparticular after a process for the production of melamine, in particularafter any desired high-pressure process for the production of melaminefrom urea, in which the melamine is initially obtained in liquid form asa melt. It is advantageous to cool the ammonia-containing melamine meltprior to solidification, in particular by supplying further ammonia atabout 1 to 50° C. above the pressure-dependent melting point of themelamine. The subsequent solidification is preferably effected in afluidized bed by bringing into contact with cold solid inert substancesor solid melamine. The method according to the invention canadvantageously also be carried out after any other desired working-upsteps of high-pressure processes. These working-up steps include inparticular:

a) separation of the NH₃/CO₂ gas mixture (off-gases) obtained in themelamine synthesis of the liquid melamine,

b) reduction of the CO₂ dissolved in the melamine by introduction of NH₃(stripping),

c) allowing the liquid melamine to dwell in the presence of ammonia(ageing),

d) cooling and solidification of the melamine, for example with water,with aqueous melamine-containing solutions or suspensions, with coldgases, such as gaseous ammonia, with liquid ammonia or with cold solidinert substances or solid melamine according to WO99/38852, for examplein a fluidized bed. The melamine melt can, for example according toWO97/20826, be sprayed into a cooling container in which an ammoniaatmosphere is present.

The purification method according to the invention is carried out afterthe solidification step, and the number of other preceding working-upsteps carried out can be varied according to the respectivecircumstances. The working-up of the melamine obtained from urea by ahigh-pressure process, up to the solidification step, may thus compriseany desired combination or only one of these steps.

According to the invention, the purification of melamine of any purityis possible. The purity of a crude melamine from a melamine productionprocess is, for example, 94-98% by weight, depending on the melamineprocess used, the melamine being contaminated in particular with melam,melem and ammelide. It is however also possible according to theinvention further to purify more highly impure melamines as well asmelamines of higher purity which have already been purified by otherpurification methods. It is possible to obtain melamine having a purityof up to more than 99.9% by weight, in some cases more than 99.99% byweight, depending on the pressure, temperature, dwell time and initialpurity used.

EXAMPLES

The starting material used in each case was a melamine which originatedfrom a high-pressure pilot plant for the production of melamine fromurea and had a purity of 98.6% by weight and was contaminated inparticular with 1.3% by weight of melam and about 0.1% by weight offurther byproducts, such as, for example, melem or ammelide(corresponding to Example A in Table 1). The analytical determination ofmelam, melem and ammelide was carried out by means of HPLC.

Examples 1 to 5

Melamine and the amount of ammonia required for achieving the desiredpressure were introduced into a laboratory autoclave. Thereafter, theautoclave was heated to a temperature of 340° C. or 330° C. and left todwell at the respective pressure for 60 min. The autoclave was thenrapidly cooled to 280° C. with water, the pressure was relieved and themelamine was analyzed by means of HPLC. The respective pressures andtemperatures during the tempering and the content of byproducts (melam,melem, ammelide) in the starting melamine (Example A) and in themelamine after the tempering (Examples 1-5) are listed in Table 1.

TABLE 1 Melamine after tempering for 60 min Pressure Temp. Melam MelemAmmelide Example (bar) (° C.) (ppm) (ppm) (ppm)  A* — — 12630 431 155 110 340 323 177 — 2 20 340 443 319 — 3 40 340 131 252   39 4 70 340 58122 — 5 100 330 105 162 — Limit of detection 2 ppm *Starting melamine

What is claimed is:
 1. Method for purifying melamine, characterized inthat impure melamine is brought into a temperature range T_(T) above325° C. and lower than the melting point of melamine which depends onthe respective prevailing ammonia pressure and is kept in thistemperature range at an ammonia pressure of from 1 to 150 bar for theduration of from 1 minute to 20 hours, the melamine being present inthis temperature range as a solid, after which, in any desired order,the pressure is relieved, cooling is effected and pure melamine isobtained in solid form.
 2. Method according to claim 1, characterized inthat impure liquid melamine is brought into the temperature range T_(T).3. Method according to claim 1, characterized in that impure solidmelamine is brought into the temperature range T_(T).
 4. Methodaccording to claim 1, characterized in that the impure melamine isbrought into a temperature range which is between 330° C. and themelting point of melamine which depends on the respective prevailingammonia pressure.
 5. Method according to claim 1, characterized in thatthe ammonia pressure is between 5 and 100 bar.
 6. Method according toclaim 1, characterized in that the melamine is kept in the temperaturerange T_(T) for a duration of from 5 minutes to 10 hours.
 7. Methodaccording to claim 1, characterized in that the method is carried outafter a process for the production of melamine.
 8. Method according toclaim 1, characterized in that the method is carried out after ahigh-pressure process for the production of melamine from urea, in whichliquid melamine, optionally after being allowed to dwell in the presenceof ammonia (ageing) is solidified in a fluidized bed by being broughtinto contact with cold solid inert substances or solid melamine and isthen kept in a temperature range T_(T) above 325° C. and lower than themelting point of melamine which depends on the effective prevailingammonia pressure, at an ammonia pressure of from 1 to 150 bar for theduration of from 1 minute to 20 hours, after which, in any desiredorder, the pressure is relieved, cooling is effected and pure melamineis obtained in solid form.